1. Field of the Invention
The present invention relates to an inside diameter measuring tool.
2. Description of Related Art
An inside diameter measuring tool called as a Holtest has been known as a device for measuring an inner diameter of an object to be measured. The inside diameter measuring tool includes: a body; an axially moving spindle having a screw to be screwed to the body; a plurality of contact pieces provided on the body, the contact pieces advancing and retracting in a direction substantially orthogonal to the axial direction of the spindle; and a conical member interposed between the contact pieces and the spindle, the conical member moving the contact pieces in the direction substantially orthogonal to the axial direction of the spindle in accordance with the axial movement of the spindle (see, for instance, Document 1: JP-U-60-41811).
Typical inside diameter measuring tools are configured so that the lead of the screw provided on the spindle is 0.5 mm and the cone angle α of the conical member is approximately 53 degrees in order to equalize the displacement of the contact pieces and the displacement of the spindle. Specifically, displacement of one of the contact pieces is represented by (displacement of the spindle)×(tan(α×½). Since tan(53×½) is approximately equal to 0.5, the displacement of the contact pieces (total displacement of two contact pieces among the plurality of contact pieces) and the displacement of the spindle can be equalized.
However, since the lead of the screw of the spindle of the inside diameter measuring tool is 0.5 mm, when an operator measures objects of various size, he has to undergo a number of rotation of the screw of the spindle in order to advance and retract the contact pieces, resulting in deteriorated operability and operation efficiency.
One solution for the above problem is to enlarge the cone angle of the conical member. When, for instance, the cone angle is 90 degrees, the contact pieces can be advanced/retracted by approximately double distance even with the same number of rotation(s) of the spindle as that of the typical tool.
However, in accordance with the increase in the cone angle, the load on the slide surface of the contact pieces applied when the contact pieces slide on the conical surface of the conical member increases. Consequently, the frictional force caused on the portion on which the contact pieces slide on the conical member increases as compared with the typical arrangement, which is likely to deteriorate the durability of the contact pieces and the conical member. Further, the displacement of the spindle is transmitted to the contact pieces at the same ratio in the typical arrangement. However, when the cone angle of the conical member is increased, the displacement of the spindle is magnified when being transmitted to the contact pieces. In other words, the contact pieces is greatly displaced for a relatively small amount of displacement of the spindle, which lowers the measurement accuracy.